1. Field of the Invention
The present invention relates to a motor for generating a rotation driving force using a shape memory alloy, a motor device, and a lens drive mechanism incorporated with the motor for use in an image sensing apparatus.
2. Description of the Related Art
The shape memory alloy (SMA) is generally used for an actuator which is actuated in response to a change of ambient temperature. For instance, the SMA is used for a valve portion to regulate the temperature of hot water in a hot water system, or for the purpose of varying a ventilating opening in accordance with a temperature change of external air. In the above usage, it is often the case that the SMA is molded into a coil spring, which is used in combination with a rectilinear reciprocating mechanism.
Japanese Unexamined Patent Publication No. 7-327380 discloses an SMA actuator which is designed to generate heat with its own Joule heat by energization thereto for actuation. As an example of the SMA actuator, there is disclosed an electric motor incorporated with rotary actuators, wherein the linear type SMA actuators are provided. Specifically, the electric motor adopts a crank mechanism, in which one ends of the substantially U-shaped SMA actuators are circumferentially and equidistantly supported on a disk-like base block, and the other ends thereof are circumferentially and equidistantly supported on a rotary drive member, which is rotatably mounted on a rotor at an eccentric position of the rotor. As the SMA actuators are sequentially expanded and contracted, the distance between the other end of the corresponding SMA actuator, and the rotary drive member is varied, thereby rotating the rotor.
In the above conventional crank mechanism, a displacement amount of each SMA actuator is significantly small, and a sufficient pivotal rotation of the rotor is difficult by the mechanism having the small number of SMA actuators as proposed in the conventional arrangement. Also, if the number of the SMA actuators is decreased, the crank diameter of the crank mechanism is decreased, which means generation of a small torque. The increased number of the SMA actuators leads to generation of a large torque, however, it may lower the rotation number of the rotor, make the assembling of the crank mechanism complicated, and raise the production cost. Increasing the axis diameter of the SMA actuator may avoid complex assembling of the mechanism to some extent. However, a required response time may be unduly extended in view of a relation between the volume and the surface area of the SMA actuators, with the result that the rotating speed of the rotor may be lowered, as compared with a case where an increased number of SMA actuators is used. Also, increasing the length of the SMA actuator may increase the size of the actuator itself.
In the electric motor employing the rotary actuators, a crank rod is provided at a center of rotation of the rotor. Accordingly, it is impossible to arrange the electric motor coaxially with a lens optical system, considering driving of the lens optical system, and it is necessary to dispose the electric motor on the side of a lens barrel, for instance. This may lead to increase the size of an image sensing apparatus incorporated with the electric motor.